Tire uniformity machines and their use and function in the production and testing of vehicle tires are well-known in the art. One of the more important tests in this regard is the detection of so-called bulges, or depressions on the tire sidewalls. Bulges and depressions are indicative of tire construction faults which can affect the safety of the end user. The primary difficulty in the detection of such fault features lies in the fact that the areas under test include intentional sculptural features, such as raised relief-type letters, and acceptable process-related surface irregularities such as pin vents or mold flash that obscure the faults. The acceptable surface features are of such a size that it is sometimes difficult to distinguish them from a bulge or depression. As a result, simple threshold value decisions on the basis of measured form data does not lead to the clear distinction between fault features and acceptable surface features.
It will be appreciated that the manufacturer of the vehicle tire may find faults in the inner construction of the tire which significantly affect the mechanical properties and thus the reliability of the tire. Construction faults show up as bulges or depressions on the exterior tire surface. An open splice in one of the construction plies, for example, is visible as a bulge. Such tires must be removed to ensure the safety of the end user. The construction faults are most likely to be found on the sidewalls and thus are superimposed on the sidewall surfaces of the tire. Accordingly, the lateral and vertical dimensions of such a bulge or depression are comparable with the lettering or other acceptable surface features which are also present. Although various methods have been developed to detect these bulges and depressions, they have been found to be lacking.
One particular attempt to address this problem is disclosed in U.S. Pat. No. 6,539,789 which employs the use of a laser fan beam that is diffusely reflected by the tire sidewall and recorded by a camera having an image sensor. By utilizing a sophisticated set of optics, the diffusely reflected radiation is quantified and from the known geometry of the measurement arrangement together with position of the projected line of the sensor surface, the form of the illuminated surface is then calculated in a manner known in the art. In particular, the system disclosed in the '789 patent employs a low pass filter applied to the structures and irregularities. This filter has the effect of suppressing the height of structures which have edges which appear as high frequency characteristics. This surface is then normalized to a quasi-planar state by subtracting the torus form of the tire. This is necessitated by the processing algorithm which works in a planar geometry. Next, all values representing height that exceed a certain threshold value are considered to contribute to an area if that area is big enough, it is then considered to be a bulge and the tire is marked as defective. Depressions can be similarly detected except for the fact that the threshold value is a negative value. Although the foregoing method is somewhat effective it is believed that it has certain drawbacks. In particular, in view of the minimal difference between the size of bulges ranging about 0.012 inches in size, and the lettering of the tire ranging about 0.040 inches in size, it is still quite difficult to distinguish between the two and correctly designate a tire as having a bulge or depression in certain instances. Therefore, there is a need in the art for improved measurement of structural defects and for processing the measured information in a manner that more reliably designates the detection of bulges and depressions in tire sidewalls.